Automobile underbody members such as suspension arms and other suspension members have heretofore been produced by forming a hot-rolled steel sheet into a predetermined form according to press-forming or the like, then assembling the formed sheets by arc-welding and thereafter coating the welded product according to cationic electrodeposition coating for application to practical use. However, at around the weld beads and the weld bead toes, Fe scale forms on the surface of the steel sheet owing to the welding heat input during arc welding. Owing to the vibration of running automobiles, the scale may peel away due to fatigue along with the cationic electrodeposition coating, and in such a case, corrosion may progress from that site to cause plate thickness reduction. Accordingly, strength designing of underbody members must include plate thickness reduction owing to corrosion, and heretofore, from the viewpoint of collision safety, hot-rolled steel sheets having a thickness of from 3 to 4 mm and having a tensile strength on a level of from 340 to 440 MPa have been much used.
Recently, further collision safety and reduction in weight have become desired, and a need of using a high-strength steel sheet having a strength of at least 590 MPa for underbody members has increased. Also, enhancement of anti-rust property is required for obtaining a long life. Further, underbody members are also required to have burring workability (hole-expanding workability) in addition to the above-mentioned high strength and corrosion resistance. Burring workability is a characteristic of indicating that the hole formed by punching or the like can be further worked with no cracking in the subsequent working process.
Patent Reference 1 discloses a high-strength galvanealed steel sheet having excellent burring workability. However, when a Zn based alloy plated steel sheet is arc-welded, then the plating layer disappears by evaporation especially at around the weld bead toes that are exposed to high temperatures, and Fe scale forms in that part. Consequently, the drawback of the heretofore-existing hot-rolled steel sheets that the coating film readily peels away along with the Fe scale could not be still overcome even by the use of Zn based alloy plated steel sheets.
Patent Reference 2 discloses a Zn—Al—Mg-based alloy hot-dip plated steel sheet, which has a higher corrosion resistance than that of ordinary hot-dip galvanized steel sheets. In this case, the metallographic structure is a dual-phase structure comprising ferrite, which is main phase, and at least one of pearlite, bainite and martensite, but the hardness difference between the two phases is great, and therefore, the plated steel sheet of the type could hardly get good burring workability.
Patent Reference 3 proposes a liquid metal embrittlement sensitivity index E value to be determined on the basis of the composition of alloying components, for the purpose of stably retarding the liquid metal embrittlement cracking in welding of a steel sheet plated with a zinc-based alloy coating. However, the present inventors' investigation revealed that, even according to the method described in this patent reference, it is not always easy to stably evade the liquid metal embrittlement cracking in welding with a Zn—Al—Mg-based alloy hot-dip plated steel sheet.